Fixes #12785

Change-Id: Iae4383889298c6a78b1ba41bd2cda70b0758fcba
Reviewed-on: https://go-review.googlesource.com/15737Reviewed-by: Andrew Gerrand <adg@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>